Method of making inductances



W. M. PANNELL I'AL METHOD 0F MAKING INDUCTANCES May 15, 1951 Filed Nov. 25. 1947 Patented May 15, 1.951A

2,552,999 METHOD oF MAKING iNnUo'iANcEs William Morgan Pannell yand Arthur Harold Shilling, Cambridge, England, assig'nors to Pye Limited, Cambridge, England, a British company Application November 25, 1947, Serial No. 787,952 In Great Britain August 31, 1946 2 Claims. 1

This invention relates to electric coils, and more especially to variable inductors comprising one or more coil windings having a metallic core or cores and forming part oi one or more tuned circuits such as are used in electric communication equipment, and particularly in tuned circuits employed in high frequency ampliiier and oscillator circuits.

, It is well known in the art that the main limiting factor in producing a large variation in inductance by effective permeability change or similar method is the ultimate thickness of the former upon which the coil is wound, since this thickness coming between the coil and its metallic core limits the minimum spacing between them, and it is known that a minimum spacing is the condition necessary to produce a maximum variation in inductance by insertion of the core. In order to produce a large variation in inductance, the former would have to be reduced in thickness almost to vanishing point, and it is obvious, therefore, that such a coil constructed according to conventional technique would have little mechanical strength.

An object of the present invention is to overcome the diiculty apparent from the foregoing, .by realising a coil construction which combines adequate mechanical strength with a capacity for inductance variation over a wide range.

From one aspect, the invention provides a coil having an adjustable core for varying its inductance, which is constructed with the coil winding or windings thereof provided externally with a body which supports the coil so that it requires little or no internal support, thereby permitting a minimum spacing between the inside of the winding or windings and the core inserted thereinto. More particularly, the invenu tion provides a coil having the winding or windings embedded in an external supporting body of a plastic or other suitable insulating material which is of suitable dimensions to provide adequate mechanical support and strength, and is provided With a hole or bore coaxial with the coil for insertion of the adjustable core; Since the mechanical `support is external to the coil winding, the hole or bore in the support may be made very little less in size than the inside dimensions of the coil winding. Consequently, the outside dimensions of the core may also be made very little less than the inside dimensions of the coil winding, as is required for producing a large change in the inductance of the coil.

According to a feature of the invention, the

coil has the supporting body moulded around it,

2 the coil thus becoming embedded in the moulded support.

Inlile manner, according to a further feature of the invention, complete tuning assemblis comprising one or more coils together with other-` components which may be required,vma'y be assembled and embedded in a supporting body of a plastic or other suitable material so as to forni a unit which may also incorporate connecting wires, leads or tags and projections, depressions or similar devices, by which external connections and xings may be made. In addition, metallic screens and cans may be mechanically support ed by, and if desired embedded in, the plastic or like body in appropriate relation to the components carried by the body.

In one embodiment of the inventiony a coil is wound on a suitable mandrel over which has been previously Wound a layer of a plastic lm or tape. The assembly is then fitted in a mould and a similar plastic injected at a suitable p oint.

The mould and mandrel are then removed leav-v-l ing the coil winding embedded in a moulded plastic body having a hole or bore therein left by the mandrel this hole or bore being (as will be appreciated) coaxial with the winding. vA metallic core of suitable material can then be passed through the hole or bore and the centre ofthe winding to, alter the inductance ofthe coil iny a manner determined by the core matef rial. The plastic lm or tape Wound on the mandrel is incorpated in the moulding and forms a thin wall von the inside of the coil winding. By varying the thickness of the plastic lm or tape, the spacing between the core and coil may be varied so as to modify the inductance change. By reducing this wall to a minimum, the change in inductance obtained permits frequency variations in a tuned circuit of approximately 31/2 to l, thus covering a 'frequency band considerably in excess of that normally obtainable with permeability tuning. t

In order that the invention may be more clearly understood and readily carried into practice, some embodiments thereof will now be described with reference to the accompanying drawings, in which K Y l v Fig. 1 is arperspective view of one" embodiment of variable inductor according to the invention, the adjustable core of the coil being shown entirely witl'idrawnV from the device, for the sake of clearness of illustration. Y

Figs. 2 and 3 are fragmentary sections diagrammaueauy uiustraung steps' in the manu'- facture of the coil, and

assaoee Fig. 4 is a perspective view oi one example of tuning unit incorporating permeability-tuned circuits and constructed according to the invention.

In the embodiment of Fig. 1, the coil winding I is embedded in a supporting body 2 of insulating material which is moulded around the winding so that the body is principally external oi the winding and supports it principally externally. 'Ihe carrier body 2 is made of a suitable plastic, such as a polythene plastic, and is dimensioned to provide adequate mechanical support and strength for the coil.

The carrier body 2 may be made of any desired external shape and provided with means for mounting it. For example, as shown, the bod;1 2 is formed with a main cylindrical portion 3 in which the winding I is coaxially embedded, and an integral plate portion 4 for mounting the dev1ce.

The cylindrical portion 3 has a cylindrical bore 5 coaxial with the winding I to receive the adjustable permeable metallic cylindrical core of the coil. The core 6 is a sliding iit in the slidable bore 5, and the body 2 is made longer than the winding I so as to provide a supporting guide for the core 6 when the latter is withdrawn from the winding I.

' Unwound ends oi the winding I are embedded in the plate portion 4 and are brought out to terminal wires 8 protruding from the plate portion 4 for effecting external electrical connections to the coil.

In manufacturing the described coil, the winding I is wound on a suitable cylindrical mandrel 9 (Fig. 2), over which has been previously wrapped or wound a layer forming a hollow cylinder I of a plastic film or tape. The resultant mandrel assembly is fitted in ay mould (not shown) for the carrier body, and the moulding material is introduced, for instance by injection, vat a suitable point into the mould and is moulded to form the body 2. The moulding material employed may be similar to the material of the tape layer I0; for instance each may be a polythene plastic, Aor alternatively the tape and body materials may be different so long as they are capable of bonding with each other. Ir desired, a polymerisable moulding material may be employed which is polymerised in situ in the mould during the moulding process.

After the moulding process has been completed, the mould and mandrel 9 are removed, leaving the coil winding I embedded in the moulded plastic body 2, as illustrated in Fig. 3, the coaxial cylindrical bore being left in the body by removal of the mandrel. The permeable metallic core 6 can then be slid along the bore through the centre of the winding to vary the inductance of the coil in a manner determined by the core material. The cylinder of plastic film or tape I0 which was wound on the mandrel becomes incorporated in the body 2 during the moulding process and forms on the inside of the winding I a thin wall II the inner surface of which (being also the inner surface oi cylinder I) forms part of the bore 5, as illustrated in Fig. 3, but it will be `noted that the principal mechanical support Afor the winding is provided by the material of the moulded body which is external to the winding. This, therefore, allows the wall II to be made thin, so that the bore 5 is very little less in diameter than the inside diameter of the winding I, and hence the diameter of the core 6 may be made Very little less than the inside diameter of the winding. By Varying the thickness of the wall I l, which is obtained by choice of the thickness of the cylindrical tape layer I0 (which may be composed of one or more layers of the tape) to suit requirements, the minimum radial spacing between the core 6 and the inside of the winding I may be varied so as to modify the maximum inductance change produced by insertion of the core ii. By reducing this wall II to a minimum practicable thickness, the change in inductance obtained permits frequency variations in a tuned circuit of approximately 31/2 to l, thus covering a frequency band considerably in excess of that normally obtainable with permeability tuning.

In like manner, a plurality of coils or windings may be embedded in a common moulded support and, furthermore, complete tuning units may be constructed having other components as well as one or more coils all embedded in a ccmmon moulded supporting body. Fig. 4 illustrates, by way of example, one such tuning unit having two tuned circuits, each comprising a permeability-tuned variable inductor and a fixed capacitor which are embedded in a moulded supporting body I2 together with a metallic screen I3 disposed between the two circuits for electrostatically shielding them from each other. One circuit comprises a capacitor i4 and a variable inductor formed by coil I5 and its adjustable core I5, and the other circuit comprises a capacitor i? and a coil IB having an adjustable core I9, the metallic screen I3 extending between the two coils i5 and I8. In the example, the two circuits are tuned simultaneously by a cord drive mechanically coupling the two cores I6 and I9. At one end of the two cores the coupling and driving cord 29 is wound on a tuning spindle 2| which may be rotatably mounted, as shown, in a bracket 22 formed integrally at one end of the screen I3. At the other end oi the cores, the cord 2i) is trained over a pulley 23 which is housed in a recess 24 formed in the moulded body I2, the pulley being supported by a spindle 25 which is passed through and supported in bores formed in the body.

In manufacturing the described unit, the two coils I5 and I8 are Wound on mandrels over previously wound plastic tape, in the manner described with reference to Fig. 2, and the ends of the coils are connected, e. g., by soldering,

to the respective capacitors i4 and Il, which are provided with terminal wires 2G for effecting external connections to the two circuits. This assembly of the two mandrels and capacitors is placed, together with the screen and bracket member I3, 22, in the desired arrangement in a mould into which moulding material is injected or otherwise introduced to mould the body i2 about the assembly so that after the moulding process is completed and the mould and mandrels are removed, the coils, I5 and I3, the capacitors It and I'I and the screen I3 are left embedded in the moulded body I2, with the terminal wires 25 protruding from the body, as illustrated, and the bracket 22 outside the body to allow of mounting the spindle ZI. The bores 2'? left in the body I2 by removal of the mandrels allow the cores I5 and I9 and their cord drive to be tted, suitable channels communicating with the bores being formed in the body, if necessary, for passage of the cord 20 into the recess 24 where the pulley 23 is tted. The body I2 may also be formed with suitable means, such as the mounting i'langes 23 shown, for mounting the unit.

For the purpose of illustration, it has been assumed in Figs. 1 and l that the carrier bodies 2 and I2 are made of transluscent material, merely in order that the details ci construction may be more clearly shown in these gurcs. While the bodies may well be moulded in a transluscent plastic, if desired, it will be, of course, obvious that transluscency is not an essential characteristic of the carrier bodies and that thei may be made of any suitable mouldable material. It will be evident also that the embodie ments described and illustrated are but two examples of the numerous forms of permeabilitytuned devices which may be constructed in accordance with the invention.

We claim:

1. Method of manufacturing an electric coil for a permeability-tuned variable inductor, which comprises the steps of applying a thin layer of non-magnetic insulating tape to a mandrel, wind ing a coil of Wire on the mandrel over said layer, moulding around said mandrel over said layer and said coil a body of mouldable nenn-magnetic insulating material bondable with said layer,

and withdrawing said mandrel to leave a bore r through said body coaxial with said coil for receiving a slidable magnetic core for varying the inductance of said coil.

2. Method of manufacturing an electric coil for a permeability-tuned Variable inductor, which comprises the steps of applying a thin layer of non-magnetic insulating tape to a mandrel, Winding a coil of wire on the mandrel over said layer, moulding around said mandrel over said layer and said coil a mouldable nonmagnetic insulating material bondable with layer to form an insulating body of substantial radial thickness externally of said coil, and Withdrawing said mandrel to leave said coil embedded in and supported by said body in close radial proximity to a bore, left by Withdrawal ci said mandrel, through said body coaxial with said coil for receiving a slidable magnetic core for varying the inductance of said coil.

V/'LLIAlx/I MORGAN PANNELL.

ARTHUR, HAROLD S-HLLNG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,589,692 Hiler June 22, 1926 1,661,953 Mcntosh Mar. 6, 1928 1,779,242 Laubi Oct. 21, 1930 2,130,741 Kronquest Nov. 29, 1938 2,144,353 Weis Jan. 17, 1939 2,179,257 Golaviznin Nov. 7, 1939 2,190,048 Sinnnger Feb. 13, 1940 2,343,999 Putnam Mar. 14, 1944i 2,402,903 Massey et al June 25, i946 2,435,630 Ketcham Feb. 10, 1949 2,459,605 Warnken Jan. 18, 1949 

